Plastic containers have been increasingly used in packaging applications, such as food packaging, and in particular for packaging convenience foods, for example ready-prepared ovenable meals which may be warmed either in a microwave oven or in a conventional oven or in either. A container which is suitable for use either in a microwave oven or a conventional oven is generally referred to as “dual-ovenable”. Often the plastic container is an APET/CPET tray (a composite material having an amorphous polyethylene terephthalate layer on top of a crystalline polyethylene terephthalate layer). Polystyrene and polypropylene containers have also been used. The plastic container is generally used in association with a lid which seals the container in order to prevent leakage and drying out of the packaged contents during storage. In addition, the lid should not stick to the packaged contents and should be able to withstand the heat generated in the oven. Such container lids normally comprise a multi-layer film, often referred to as a “lidding film”, which comprises a flexible polymeric substrate, such as biaxially oriented polyester film, and a heat-sealable coating layer.
The manufacture of sealed containers using lidding films involves the formation of a seal between the lidding film and the container. This seal is formed by placing the lid on top of the container and applying heat and pressure in order to soften or melt the sealable coating layer so that it adheres to the surface of the container and forms an effective seal between the lid and the container. The seal must be strong enough to prevent leakage of the contents. The film lid should be peelable from the container by the consumer and in such cases the seal must be strong enough to prevent leakage of the contents but not too strong to result in difficulties in removing the lid when the container is to be opened. In particular, the lid should not tear during peeling, which could result in portions of the film lid falling into the contents of the container thereby spoiling the food. A strong seal and easy-peeling properties, i.e. a clean peel, may be required at both low, e.g. ambient, and high temperatures, e.g. after heating the packaged food contents in an oven.
In addition, when disposing a food product in the container to be sealed, solids or liquids from the food product or other contaminants may come into contact with, and remain on, the top surface of the lip of the container which is to be sealed to the lidding film. This may result in poor seal properties between the container and the lid, and eventually a weak overall package. A film which performs well in this respect, i.e. which shows good seal properties despite the presence of contaminants between the lid and the container, is said to exhibit good “seal through contamination”, and this is a further desirable property for these heat-sealable lidding films. In conventional lidding films, this problem is typically addressed by increasing the thickness of the heat-seal layer, for instance, to about 25 μm or above, which is economically disadvantageous.
A further desirable sealing property is the characteristic of good “hot-tack” adhesion. This property essentially measures the speed at which a heat-seal bond is formed when a heated and softened (or molten) heat-sealable film is contacted with the surface to which it is to be sealed. Hot-tack adhesion essentially therefore corresponds to the property of heat-bond seal strength, but wherein hot-tack adhesion is measured after a much smaller time interval (typically 0.1 second) after the heat-seal bond is initiated. Heat-seal bond strength is measured once the heat-seal bond has completely formed, and normally after the heat-seal bond has cooled to ambient temperature, and can be referred to as the “cold heat-seal bond strength”. Good hot-tack adhesion is important for quick, efficient and reliable packaging. In addition, in circumstances where the food to be packaged is sufficiently bulky that it protrudes above the lip of the container, the formation of a rapid heat-seal bond is desired. Typically, hot-tack adhesion is approximately proportional to the cold heat-seal bond strength, but while it is desirable to maximise hot-tack adhesion, if the hot-tack adhesion is too high then the cold heat-seal bond strength may be too strong to allow an easy and clean peel. In general, the hot-tack adhesion increases as the molecular weight of the heat-sealable polymer increases. Many thermoplastic polymers display hot-tack adhesion to some degree, albeit at different temperatures and viscosities.
In many prior art lidding films, the heat-sealable layer is applied to the substrate using either an organic solvent, or an aqueous dispersion or solution. The use of organic solvents is generally disadvantageous because they may be harmful, hazardous in use, or toxic and detrimental to the environment. In addition, films made in this way often contain a residual amount of solvent, and so may not be suitable for use in applications where they come into contact with food products. The use of organic solvents usually involves an “off-line” coating step, i.e. after any stretching and subsequent heat-setting operation employed during the manufacture of the substrate, since such solvents can cause sticking or blocking of the film during the normal winding operations used during film manufacture. The use of an aqueous dispersion or solution, such as in the process of WO-A-96/19333, avoids the use of substantial amounts of organic solvents; allows the use of a more efficient “in-line” coating process, i.e. wherein the coating layer is applied either before the film substrate is stretched or between the stretching steps of a biaxial stretching process; but is limited to coating compositions which are soluble or adequately dispersible in water. An in-line process avoids the use of the further heating or drying steps which are encountered in off-line coating processes, particularly off-line solvent-coating processes. These processes can embrittle the film and deteriorate the tensile properties. In general, an in-line coated film has therefore superior mechanical properties.
Heat-sealable films have also been manufactured by other in-line coating techniques. For instance, GB-2024715 discloses the application of a polyolefinic material onto a polyolefinic substrate using an extrusion-coating technique between the longitudinal and transverse stretching operations (“inter-draw” coating). A process for the in-line inter-draw extrusion-coating of polyolefins onto a polyester substrate to produce a heat-sealable film is disclosed in GB-1077813. U.S. Pat. No. 4,333,968 discloses a method for the inter-draw extrusion coating of an ethylene-vinyl acetate (EVA) copolymer onto a polypropylene substrate to provide a heat-sealable peelable film.
It is an object of the present invention to address one or more of the aforementioned problems and provide an improved and more economical packaging means for a ready-prepared ovenable meal. It is a further object of this invention to provide a heat-sealable and peelable film suitable for use as a packaging means for a ready-prepared ovenable meal.